Direction of theoretical and experimental investigation into the mechanism of n-HA/Si-PA-SC@Ag as a bio-based heterogeneous catalyst in the reduction reactions

In the present study, a natural-based heterogeneous catalyst is synthesized. For this purpose, nano-hydroxyapatite (n-HA) is prepared, silica-modified and functionalized with phthalimide. Finally, Ag2+ was immobilized onto n-HA/Si-PA-SC and reduced to Ag nanoparticles by Bellis perennis flowers extract. n-HA/Si-PA-SC@Ag characterized by TGA, FTIR, SEM/EDX, XRD, TEM, BET and ICP-AES techniques. Moreover, metal–ligand interactions in n-HA/Si-PA-SC@Ag complex models were assessed to make a quantitative representation for the immobilization behavior of Ag NPs on the surface of n-HA/Si-PA-SC through quantum chemistry computations. Furthermore, the performance of n-HA/Si-PA-SC@Ag was studied in the nitroarene, methylene blue and congo red reductions. Finally, the recyclability study as well as Ag-leaching verified that, n-HA/Si-PA-SC@Ag was stable and reused-up to four times without losing its activity.

www.nature.com/scientificreports/ use in the important and widely used reduction reaction, for which various metals are used. Ag NPs are a suitable option for these reactions due to their unique properties, especially their affordability and availability 15,[19][20][21][22][23] . Nitroarene compounds, which can be said to be relatively rare in nature, have entered the environment and caused pollution through human activities such as agriculture, dyeing, and some factories. Among them, nitrophenols are often known as toxic compounds of environment pollutants that they can easily affect life by contaminating sewage and food chain system 24,25 . In the environmental point of view, the synthesis of amines through the nitro-reduction process is one of the motivating reactions 15,[26][27][28] . Organic reactions, especially the hydrogenation of nitro compounds, are very important in aquatic environments, and due to the lower-cost and ecological pollution, as well as higher-safety, they form the basis of some environmentally friendly research 15,29 .
On the other hand, dyes as key materials in textile, food, paper, food industries and pharmaceutical lead to environmental pollution, especially water wastage 30,31 . Therefore, the industrial effluent's control is essential to create a clean environment. Methylene blue (MB) as a cationic substance and Congo red (CR) as an anionic dye are extensively applied in industries for example, rubber, plastic and paper, which harm the environment if not cleaned in time 32 . Therefore, due to the importance and preservation of the environment, it is required to develop a simple manner for the efficient decomposition of dyes, which is expected to be achieved by Ag NPs because of their relatively large surface-to-volume ratio 33 .
In line with our investigations on the design, synthesis and computational modeling of heterogeneous catalysts and development of ecologically benign methods for chemical synthesis [34][35][36][37][38][39] regarding the reports of researchers in the field of joining theory and experiments [40][41][42] , we have recently focused on the application of naturalheterogeneous catalysts in several organic conversions 19,[43][44][45] . Hence, we are introducing efficient catalysts using n-HA decoration with organic functionalities and Ag-NPs doping by bio-assisted method. On the other hand, a quantitative description for metal-ligand interactions in n-HA/Si-PA-SC@Ag complex models is assessed by performing theoretical calculations using density functional theory to interpret the deposition of silver nanoparticles on the nano-rod hydroxyapatite support. Finally, it can be acknowledged that n-HA/Si-PA-SC@Ag has been used as a recoverable and heterogeneous nanocatalyst in the reduction reaction of nitroarene compounds as well as MB and CR with excellent yields.

Result and discussion
Catalyst characterization. After successful synthesis of the n-HA/Si-PA-SC@Ag, the catalyst structure was performed by several analyzes. It should be noted that the interpretation of FTIR, XRD, BET, TGA and SEM/ EDX analyzes is described in SI.
The morphological characteristics including the shape and particle size of n-HA/Si-PA-SC@Ag were investigated using TEM analysis (Fig. 1). According to the results obtained from SEM analysis, the nano-rod structure of n-HA is clearly visible. In addition, the uniformly distributed black dots on the surface of the n-HA/Si-PA-SC are a confirmation of the successful stabilization of Ag nanoparticles, which is consistent with the EDX results and XRD patterns. It should be noted that the average diameter of Ag nanoparticles is ~ 14 nm that is practically consistent with the result from the Debyee-Scherrer Eq. (10.9 nm). Finally, it can be acknowledged that this method has succeeded in effectively stabilizing Ag nanoparticles on the surface of n-HA/Si-PA-SC.

Computational section.
Recently, we have concentrated on the computational modeling of incorporation of transition metal nanoparticles (NPs) on a wide range of functionalized heterogeneous catalyst supports, in combination with the experimental assessments on their synthesis and applications in different organic reactions. In this line, we have investigated the immobilization behavior of palladium and copper NPs on the modified poly (styrene-co-maleic anhydride) surface. In addition, immobilization of Cu nanoparticles on the aminated and N-sulfamic-aminated KIT-5 nanocatalysts, poly(methyl methacrylate-co-maleimide) support and various functionalized halloysite nanoclays were modeled using quantum chemistry approaches 34,37,[46][47][48][49][50][51][52] . In the recent year, we have assessed metal-ligand interactions in an appropriate designed model of n-hydroxyapatite supported-silver catalyst, functionalized with 4-aminoacetanilide 39 . In continuation of the above-mentioned joint experimental and computational researches, in this work, we have presented a reliable structural model of n-hydroxyapatite surface, functionalized with phthalimide and semicarbazide to investigate the complexation behavior of silver NPs in this heterogeneous nanocatalyst. It is significant to note that in order to present a computationally feasible approximation of large systems such as the surface of heterogeneous catalysts, periodic boundary conditions are often applied using unit cells as modeling boxes. During the computations only the properties of the original unit cell are calculated and propagated in the chosen dimensions. In this research, since we have investigated the immobilization behavior of AgNPs on the catalyst surface, we focused on the interaction of silver nanoparticles with phthalimide and semicarbazide segments of functionalized surface. So, we designed the complex models of Ag@n-HA@SiO 2 -PA-SC and performed non-periodic DFT computations, regardless of the whole surface of catalyst. As the first step, we have illustrated the proposed structural design of complex model (denoted as n-HA/Si-PA-SC-Ag) together with the possible coordination modes (Fig. 2). It should be noted that from the time and efficiency viewpoints in computational process, the suggested complex size has a reliable synchrony 36,53 . The optimized geometry of n-HA/Si-PA-SC ligand and n-HA/Si-PA-SC-Ag complex have been depicted in Fig. S6 which have been obtained through density functional theory (DFT) calculations at M06/6-311G** level 54 . Moreover, in Fig. S6, we have displayed the calculated values of bond order (together with the bond length in the parenthesis) for some key bonds in the coordination modes of n-HA/ Si-PA-SC-Ag complex model. It is essential to emphasize that M06 functional has been classified as a highly parametrized exchange-correlation hybrid functional with meta-generalized gradient approximation that aims for a balanced description for both main-group and transition-metal chemistry. www.nature.com/scientificreports/ theory for various databases, including thermochemistry, kinetics, noncovalent interactions, transition metal bonding, metal atom and molecular excitation energies, bond lengths, vibrational frequencies, and vibrational zero point energies. In order to verify the optimization procedure, we examined all real frequencies and all DFT computations have been performed using GAMESS suite of programs 55 . As it can be clearly extracted from Fig. S6 the calculated bond order values of selected N-N, C-O and C-N bonds have been fallen through metalligand interactions which can be directly due to the donation of shared electrons from this chemical bonds to silver atoms, which are obviously validated with our obtained FT-IR elucidations.
In the next step, we employed the quantum theory of atoms in molecules (QTAIM) methods 56,57 to analyze the topological properties of electron densities at the optimized structure of n-HA/Si-PA-SC ligand and n-HA/ Si-PA-SC-Ag complex. To this end, we used the calculated M06/6-311G** wave function of the optimized geometry of n-HA/Si-PA-SC ligand and n-HA/Si-PA-SC-Ag complex as input files for AIM2000 program package 58 . In Fig. 3 we presented the QTAIM graphs of n-HA/Si-PA-SC ligand and n-HA/Si-PA-SC-Ag complex that demonstrate all bond and ring critical points and bond paths. Furthermore, we calculated the various QTAIM indicators such as electron density (ρb), its laplacian (∇ 2 ρb), electronic kinetic energy density (Gb), electronic potential energy density (Vb), total electronic energy density (Hb) and the ratio of |Vb |/Gb which have been reported in Table S2, for some selected key bond critical points (BCPs) and ring critical points (RCPs) in n-HA/ Si-PA-SC ligand and n-HA/Si-PA-SC-Ag complex.
The reported results of Table S2 reveal   www.nature.com/scientificreports/ clearly affirm the partially electrostatic-covalent character of metal-ligand interactions in this model complex. Another significant aspect of QTAIM calculations can be concluded via the more precise analysis of QTAIM molecular graphs which portrays various intramolecular BCPs between silver atoms with nitrogen, oxygen, carbon and hydrogen atoms of n-HA/Si-PA-SC ligand that generates some new ring critical points (RCPs) and therefore, leading to the considerable electronic stabilization impact on the complexation procedure.  www.nature.com/scientificreports/ Catalytic activity. Our goal in this project was the synthesis of a nanocatalyst that can effectively reduce nitro compounds and be cost-effective with a heterogeneous and recyclable nature. Therefore, first n-HA was prepared and after successful characterization, it was investigated in the reduction reaction of p-NP. For this purpose, First, the catalytic study of the n-HA in the reduction of p-NP (0.5 mmol) to its relevant p-AP over NaBH 4 (7.5 mmol) and 30 mg of catalyst as a model reaction was preferred. Based on the obtained results, no amount of catalyst was able to promote and complete the reduction in the presence of NaBH 4 amounts. As mentioned, our intention was to synthesize a recyclable catalyst, but after separating the bare n-HA catalyst from the reaction mixture, its semi-heterogeneous nature was revealed and more than half of it lost during the separation process. Therefore, to improve the catalytic activity and catalyst recovery process, the n-HA substrate was functionalized in several steps and in each stage its catalytic activity was investigated according to the above conditions. As expected, the use of SiO 2 to aid heterogeneity was beneficial but had no effect on the reduction reaction process. Next, in order to understand the reason of what factors in the structure of the catalyst can play a role in the reduction process, the silver salt (AgNO 3 ) that was available was used as a catalyst. Surprisingly, p-AP was obtained after 4 h with 70% yield but its separation and recovery were not sufficient for our purpose. In the following, organic and inorganic materials were used to functionalize the n-HA substrate and prepare the linker containing heteroatoms to load more silver metal. As tabulated, the functionalized n-HA substrates were incapable of reducing the nitro compound, but they created a strong interaction for Ag loading and preventing its leaching. Importantly, no products and no noticeable color changes were observed in the reactions of the metal-free catalysts (see Fig. S7 in SI.). It should be mentioned that the metal-free catalysts are not operative in reducing p-NP. This observation confirmed that p-NP reduction reaction was possible in the presence of Ag nanoparticles. By comparing the results of AgNO 3 salt and n-HA/Si-PA-SC@Ag catalyst, it can be stated that AgNO 3 salt are able to promote the reaction and its difference with n-HA/Si-PA-SC@Ag catalyst is in recovery and separation (Table S3, entries 12-16). Actually, n-HA/Si-PA-SC@Ag catalyst was easily separated by simple filtration from the reaction mixture and was recycled up to four times with high yields, while AgNO 3 salt was difficult to separate and the investigation of recyclability was impossible. Based on these observations, it can be concluded that using the current protocol is effective in stabilizing Ag particles as an active site in the catalyst structure via bio-based pathway. For further study, the catalytic properties of the n-HA/Si-PA-SC@Ag was expanded and optimized in the reduction reaction of NAs, MB and CR with NaBH 4 . One of the most important of p-NP reduction is detection of the reaction product that the progress can be followed by detecting the changes in UV-Vis absorption at 400 nm and 300 nm. Based on the result of the model reaction, after 5 min the color of reaction changed to colorless from yellow (Fig. 4a). Immediately, catalyst was separated and a solution with certain molarity was prepared and its UV-Vis absorption was investigated. As presented in Fig. S7, the absorption peak at 400 nm was removed and a sharp peak at 300 nm demonstrating the presence of p-AP was appeared. Then, to select the best conditions for reduction of p-NP, the amount of H 2 O as a media, catalyst and NaBH 4 were optimized and all results are tabulated (Table S3 and Fig. S8, S9, S10). Next, diverse amount of NaBH 4 using n-HA/Si-PA-SC@Ag (20 mg) in the reduction of p-NP (0.5 mmol) in water (5 mL) at r.t. were studied (Table S3, entries 1-5 and Fig. S8). According to Fig. S8, the peak at 400 nm significantly reduced from 2.5 mmol (50 min) to 10 mmol (5 min) of NaBH 4 and a new peak corresponds to p-AP was observed. This result indicates the reduction of p-NP produced absolutely p-AP, without any by-products with 100% yields while when 2.5 mmol of NaBH 4 was used, only the p-AP was obtained with 40% yield. Based on experimental results from optimization of catalyst amount, almost similar times for p-NP reduction reaction were observed for 30 and 40 mg of n-HA/Si-PA-SC@Ag (Table S3, entries 6-9 www.nature.com/scientificreports/ and Fig. S9). Time-dependent changes in the absorption peak of p-NP at 400 nm occurred over n-HA/Si-PA-SC@Ag. The results of the reduction procedure in the absence of n-HA/Si-PA-SC@Ag and NaBH 4 are presented in Table S3 and Fig. S10. As expected, color changes and desired products were not obtained without the use of n-HA/Si-PA-SC@Ag and NaBH 4 over 2 h. We also established the catalytic activity of catalyst for the reduction of other nitro-substates (Table 1). As tabulated and exhibited in Fig. S11 and based on mechanism in Fig. S12, it was found that our n-HA/Si-PA-SC@ Ag catalyst promoted high reactivities for several nitroarenes bearing electron-donating and withdrawing groups and nitrobenzene. Notably, when p-nitrophenyl palmitate was used as a substrate, it not only did not lead to reduction, but also caused the breakdown of the bond between oxygen and carbonyl group and the production of p-AP (Table 1, entry 6). It should be mentioned that the structure of some products were confirmed by GC analysis. (see Figs. S18, S19,S20, S21 and S22in SI.).
The reductive conversion of p-NP to p-AP is a six-electron transfer reaction in the presence of NaBH 4 as a reducing agent, but will not proceed well in the absence of a catalyst. According to the reported mechanisms 59 , NaBH 4 first produces nitro-phenolate ions, then BH 4 (borohydride) and C 6 H 4 NO 3 − (p-nitro-phenolate) ions are absorbed on the catalyst surface for electron transfer, and nitrophenolate ions absorb at 402 nm is significantly reduced and the reaction mixture is colorless. The reaction mechanism of the conversion of p-NP to p-AP in the presence of n-HA/Si-PA-SC@Ag is depicted in Fig. S12.
Another possible application of synthesized n-HA/Si-PA-SC@Ag catalytic activity was the reduction of MB to LMB and CR to sodium-4-amino-1-naphtalene solfunate by NaBH 4 . For this porpuse, the reaction of MB or CR (0.5 mmol) over NaBH 4 (10 mmol) and 300 mg of n-HA/Si-PA-SC@Ag in water at r.t. was started and the reaction progress followed by UV-Vis spectrophotometry in 400 and 800 nm. At the start of the reaction, the MB solution showed two peaks 664 and 614 nm 60 . After a few minutes of the reaction, these two peaks gradually decreased until after 2 min they completely disappeared and the color of reaction changed from blue to colorless, while this mixture did not change after two hours in the absence of n-HA/Si-PA-SC@Ag catalyst ( Table 2, entry 1). The UV-Vis spectrum of the MB reduction by NaBH 4 over n-HA/Si-PA-SC@Ag catalyst is shown in Fig. 5a. In agreement with proposed mechanism, the reduction process was establish to be enhanced over Ag nanoparticles and also exhibited a fast decrease in the absorption intensity of MB solution. In fact, Ag nanoparticles help in the electron relay from BH − 4 BH 4 − as a nucleophilic core to MB as an electrophilic core (Fig. 5a) 61 . On the other side, the reduction reaction of CR (0.5 mmol) in the presence NaBH 4 (10 mmol) and n-HA/ Si-PA-SC@Ag (300 mg) in water at r.t. was investigated and the progress was checked by UV-Vis spectrophotometry in 250 and 800 nm. When the reaction was started, the CR solution showed two peaks 498 nm (π → π * ) and 350 nm (n → π * ), transition associated with the azo-group 62 . After a few minutes of the reaction, these two peaks gradually decreased until after 8 min they completely disappeared and the color reaction changed to colorless from red, while this mixture did not change after two hours in the absence of n-HA/Si-PA-SC@Ag catalyst ( Table 2, entry 2). The UV-Vis spectrum of the CR reduction over n-HA/Si-PA-SC@Ag catalyst and NaBH 4 is shown in Fig. 5c. In accordance with 63 , CR shows an absorption peaks that metal act as an electron relay, and electron transfer take place via Ag nanoparticles from BH − 4 BH 4 − as a nucleophilic molecule to CR as an electrophilic molecule. Moreover, CR mixture and BH − 4 BH 4 − ions in the presence of Ag nanoparticles was rapidly decolored representing the significant catalytic influence of Ag nanoparticles in the degradation of CR. The UV-Vis spectra of MB and CR exhibited a impressive decrease in peak strength because of the reduction by NaBH 4 over n-HA/Si-PA-SC@Ag catalyst.
Additionally, it can be inferred that the reduction reaction follows pseudo-first-order kinetics because NaBH 4 is usually consumed in excess compared to the concentration of nitrophenols and nanocatalysts. A few fundamental equations that give us information about the progress of a reaction are described below: The r, C t , C o, A t and A o parameters represent the rate of reduction, concentration of nitro compound at any time t, initial concentration at zero time, absorbance at any time t, and the initial intensity of absorbance at time zero, respectively. Based on Eq. (3) it can be mention that absorption ratio of nitro phenols is equals to that of concentration ratio in reduction medium from any time t = t to initial time t = 0. Also, the apparent rate law can be easily calculated using by Eq. (2), that kaap is the apparent rate constant for first-order kinetics 64 .
The linear plots of the reduction of p-NP, MB and CR are demonestrated in Figs. 4b, 5b and 5d. As presented in Fig. 4b, a linear correlation was found between ln (A t /A 0 ) and time and the calculated rate constant (k) was about 0.71 min -165 .The linear plot of ln (A t /A 0 ) versus time shows that the reduction reaction followed the pseudofirst-order kinetics, and the calculated rate constant (k) was 0.473 min −1 (Fig. 5b). As shown in Fig. 5d, a linear correlation between ln (A t /A 0 ) and time and the calculated rate constant (k) from the slope was 0.037 s −1 . In the absorption process, the effect of contact time in different concentrations of p-NP, MB and CR dyes (0.031, 0.051, 0.073, and 0.095 mg/L) on the catalyst (0.28 mg/L) was investigated. The test solution for p-NP was performed at different time intervals (1, 2, 4, 5, 10, 20, 30, 40 and 60 min). The absorption range of p-NP was found from 80 to 100% in the studied concentrations of 0.031, 0.051 and 0.073 mg/L (Fig. S13c) and the absorption range of www.nature.com/scientificreports/ MB and CR dyes were found from 80 to 100% in the studied concentrations of 0.031 and 0.051 mg/L ( Fig. S13a  and b) that the absorption rate was faster in MB. Based on the results obtained, the color removal rate decreased with increasing color concentration from 0.031 to 0.095 mg/L, which was the same for all three samples. The reason for that was the less availability of binding sites in blue dye solutions. In order to investigate the activity of the n-HA/Si-PA-SC@Ag catalyst as much as possible, the reduction reactions of p-NP, MB and CR were investigated in the absence of the catalyst and the results were compared with each other. As seen in Fig. S14, S15 and S16 and according to the mechanism, nitro compounds need Hfor reduction processes, which is provided from NaBH 4 salt, but the presence of Ag is very effective and acts as a catalyst to advance the reaction and complete it. This reaction is completed in the presence of hydrogen source and catalyst and is incomplete in the absence of any of these. As shown in Fig. S14, when the catalyst alone is present in the reaction mixture, we have more than 50% of the amine product, while in the absence of the catalyst, there is no reduction in the reaction mixture. Furthermore, the dependence of the reduction reaction on the hydrogen source is clearly visible in the reduction of MB and CR dyes, but the presence of the catalyst is also effective (Fig. S15 and S16).
For investigation of the further proficiencies, the performance of the n-HA/Si-PA-SC@Ag in the reduction of p-NP, MB and CR were compared with other recent reports (Table S4, entries 1-14). As tabulated, it was determined that AAs were achieved in better reaction condition and shorter reaction time by using n-HA/ Si-PA-SC@Ag (Table S4, entry 6). Actually, the biggest advantage of the above catalyst is achieving the highest yields in the shortest reaction times along with easy separation. Among other advantages of this manner we cat mention, green nature of n-HA/Si-PA-SC@Ag, eco-friendly process, easy workup procedure and high product's yield that are formed in mild conditions. Recyclability study. Based on the importance of recycling modern catalysts in their applied use, the ability of n-HA/Si-PA-SC@Ag in the synthesis of AAs through the reduction reaction was studied. For this end, upon completion of the reaction, n-HA/Si-PA-SC@Ag was filtrated, washed with H 2 O/ethanol and used for the next run under the same condition. As can be seen that in Fig. S17C, this series was repeated up to four repeated times without any decrease in activity that verified by Ag leaching results (0.0010 mmol.g −1 ). Regarding to the results obtained from Fig. S17C, the UV-Vis spectra of the products of each stage of the reduction reaction over the recycled n-HA/Si-PA-SC@Ag investigated. Surprisingly, the results confirm that p-AP was obtained with 100% yield without any side-products (Fig. S17D). As delineated in Fig. S17B, the stability of the structure of recycled n-HA/Si-PA-SC@Ag was studied using recording FTIR analysis after one and last runs. Obviously, all spectra are similar and no momentous changes were detected upon recycling, which this observation was matched with the results obtained from SEM analysis (Fig. S17A).   www.nature.com/scientificreports/ Synthesis of n-HA/Si-Cl. To synthesize the n-HA/Si-Cl, n-HA/Si (1 g) was dispersed in toluene (35 mL) for 30 min. After that, the mixture was heated at refluxed under Ar and (3-chloroprppyl) trimethoxysilane (7 mL) and Et 3 N (1.3 mL) were added into the mixture and its pH was kept at 8.5. After overnight, the resulting mixture filtered, washed with toluene and dried at 45 °C for 6 h.
Synthesis of n-HA/Si-PA. Regarding to synthesis n-HA/Si-PA, n-HA/Si-Cl (3 g) dispersed in toluene (60 mL Synthesis of n-HA/Si-PA-SC. N-HA/Si-PA (5 g) was dispersed in 70 mL toluene under U.I. for 30 min. Then, a solution of semicarbazide (5.1 g in 90 mL methanol and toluene/ 2:1) was prepared and added into the above mixture with Et 3 N (~ 2 mL) and refluxed and Ar for 24 h (pH ~ 9). Lastly, the mixture was filtered, washed with toluene/methanol and dried at 40 °C in an oven.
Plant material and extract preparation. The plant was collected in April 2011, in Siahkal, Gilan, Iran. The voucher specimen has been identified by Dolatyari, Ramezani and Ajani and deposited at the Flora of Iran Herbarium of Iranian Biological Resources Center (Collection number IBRC P1006947). The flowers (10 g) collected from Bellis perennis L., Asteraceae and then crushed in porcelain mortar and turned into a uniform powder. To the obtained powder, water (100 mL) was added and heated for 2 h at 100 °C. When the color solution becomes dark and its volume is half, the extract was filtrated and it was used for the reduction of metal salts (pH ~ 10.2).
Synthesis of n-HA/Si-PA-SC@Ag. In the last step, AgNO 3 salt was incorporated onto the n-HA/Si-PA-SC via bio-assist approach. First, n-HA/Si-PA-SC (3 g) was dispersed in H 2 O (35 mL) for 30 min and stirred under Ar. After that, AgNO 3 (0/09 g in 10 mL of H 2 O, pH ~ 7) solution was prepared and added into the above mixture and stirred for more 30 min. Then, the fresh extract (10 mL) was dropwise added and the mixture stirred for 4 h. Obviously, the color of the mixture was changed from white to black, which this observation caused by the reduction of Ag(II) salt to Ag(0)-NPs. Notably, after adding fresh extract to the mixture including AgNO 3 , the pH was increased to ~ 9. Eventually the mixture was filtered, washed with H 2 O/EtOH and dried in oven at 60 °C for 15 h and then the gray powder was achieved (Fig. 6).
Catalytic activity. General method for the reduction of NAs. For investigation of the catalytic activity, in a typical manner in a solution of NAs (0.5 mmol, in 1 mL H 2 O), a solution of NaBH 4 (7.5 mmol, in 1.5 mL H 2 O) and n-HA/Si-PA-SC@Ag (30 mg) as a catalyst were added and stirred at r.t. (pH ~ 9). The reaction process was checked by UV-Vis and TLC. Upon completion of this reduction (changing the mixture's color from yellow to colorless), n-HA/Si-PA-SC@Ag was filtrated, washed and dried for using next run. Finally, a solution (0.01 M, 5 mL) from concentrated was prepared and the purity percentage and yield of the product were checked by UV-Vis and GC analyses.
General method for the reduction of MB and CR. Two solution of MB and CR (0.5 mmol in 3 mL H 2 O) were prepared, separately. Then 300 mg n-HA/Si-PA-SC@Ag and a solution of NaBH 4 (10 mmol in 2 mL H 2 O) were added to them and both were stirred until they became colorless (pH ~ 10.5-11). Upon completion of the reduction, the reaction process was followed by UV-Vis. After completion of the reduction (changing the color of the reaction mixture blue and orange to colorless), n-HA/Si-PA-SC@Ag was filtrated, washed and dried for using next run. Finally, a solution (0.2 M, 25 mL) from concentrated was prepared and the purity percentage and yield of the product were checked by UV-Vis analysis.

Conclusion
As a whole, a natural-based and heterogeneous catalyst has been developed by the preparing and functionalizing of n-HA with phthalimide, semicarbazide and Ag nanoparticles that were incorporated via bio-assisted approach using BPE. The structure of synthesized catalyst with stability and suitability of the synthesis method were confirmed. The performance of n-HA/Si-PA-SC@Ag for the reduction of NAs, MB and CR were investigated. n-HA/Si-PA-SC@Ag catalyst is not only effective in the reduction of NAs but also shows outstanding activity in the reduction of organic dyes. Besides to its high reactivity, the above catalyst was easy separated and recycled up to four more times. Noteworthy, the use of n-HA as a solid substrate has a significant and positive effect on immobilizing Ag and suppressing its leaching. Likewise, the DFT and QAITM computational results were consistent with that of experimental observations from the structural and electronic point of view.  www.nature.com/scientificreports/